I’m a computer science student in korea, and I’m working on writing code to post data using react-form-hook, yup, and axios. I need to include JSON inside the form data to send it, but I keep getting a 400 error. Thank you for reading, and I really appreciate any small comments.

"req": {
    "username": "string",
    "password": "string",
    "nickname": "string",
    "profileTagIds": [
      0
    ]
  },
  "profileImage": "string"
}

in swager api,

this is my code

export default function SignUp() {
  const {
    register,
    handleSubmit,
    formState: { errors, isValid },
    reset,
  } = useForm({
    resolver: yupResolver(schema),
    mode: 'onChange', // onchange 당 체크
  });
  const onSubmit = async (data) => {
    try {
      const formData = new FormData();

      const file = data.file[0];
      const fileAsBase64 = await convertFileToBase64(file);

      const json = {
        req: {
          username: data.email,
          password: data.password,
          nickname: data.id,
          profileTagIds: [0],
        },
        profileImage: fileAsBase64,
      };

      const blob = new Blob([JSON.stringify(json)], { type: 'application/json' });

      // Append JSON blob with a specific key
      formData.append('json', blob);

      // formData 내용 출력
      Array.from(formData.entries()).forEach(([key, value]) => {
        console.log(key, ':', value);
      });

      const result = await axios.post(`${process.env.REACT_APP_API_URL}/member/join`, formData, {
        headers: {
          'Content-Type': 'multipart/form-data',
        },
      });
      console.log('success', result.data);
      reset();
    } catch (error) {
      console.error('fail', error.response?.data);
    }
  };

and this is 400 error :

json : 
File {name: 'blob', type:
"application/json"

At first, I was told that I had to use multi-part format data, so I put the image in that format and sent it with json, but the error kept happening. This time, I changed the image to stirng (encoded into base 64) and sent it, but I still get 400 errors.what can i do?